Automated Surface Wave Measurements for Evaluating the Depth of Surface-Breaking Cracks in Concrete
© The Author(s) 2015
Received: 2 February 2015
Accepted: 12 August 2015
Published: 2 September 2015
The primary objective of this study is to investigate the feasibility of an innovative surface-mount sensor, made of a piezoelectric disc (PZT sensor), as a consistent source for surface wave velocity and transmission measurements in concrete structures. To this end, one concrete slab with lateral dimensions of 1500 by 1500 mm and a thickness of 200 mm was prepared in the laboratory. The concrete slab had a notch-type, surface-breaking crack at its center, with depths increasing from 0 to 100 mm at stepwise intervals of 10 mm. A PZT sensor was attached to the concrete surface and used to generate incident surface waves for surface wave measurements. Two accelerometers were used to measure the surface waves. Signals generated by the PZT sensors show a broad bandwidth with a center frequency around 40 kHz, and very good signal consistency in the frequency range from 0 to 100 kHz. Furthermore, repeatability of the surface wave velocity and transmission measurements is significantly improved compared to that obtained using manual impact sources. In addition, the PZT sensors are demonstrated to be effective for monitoring an actual surface-breaking crack in a concrete beam specimen subjected to various external loadings (compressive and flexural loading with stepwise increases). The findings in this study demonstrate that the surface mount sensor has great potential as a consistent source for surface wave velocity and transmission measurements for automated health monitoring of concrete structures.
1 Introduction and Motivation
Surface wave measurements have been widely used to develop non-destructive evaluation (NDE) techniques for concrete structures in civil engineering due to their useful features (Graff 1991). Surface waves are mechanical waves that propagate along the surface of concrete with most of their energy confined near the surface, which enables one-sided access of concrete structures. The particle vibration amplitude of surface waves exponentially decreases with distance from the free surface boundary and with a frequency-dependent penetration depth, which is particularly useful to identify and characterize surface-breaking or sub-surface defects in concrete structures (Achenbach 2000, 2002). In infinite media, surface waves are non-dispersive, that is the wave velocity does not change with frequency. In practice, this assumption is valid when the thickness H of the solid body of interest is sufficiently larger than the wavelength λ of the surface wave (i.e., H > 2λ). In thin plates or layered systems, the velocity of surface wave changes with frequency. Surface wave velocity measurements have been demonstrated to be effective for characterizing mechanical properties of concrete in many civil engineering applications (ACI committee 228 1998).
A possible solution to the aforementioned problems can be obtained by using “surface-mounted piezoelectric transducers (PZT)”. PZT has been successfully applied to the structural health monitoring of concrete structures by using stress wave-based methods (Liao et al. 2011; Okafor et al. 1996; Song et al. 2006). The wave propagation properties were studied to detect and evaluate the cracks and damages inside concrete structures. Wang et al. (2001) studied the debonding behavior between steel rebar and concrete by using PZT (lead zirconate titanate) patches fixed to generate and receive elastic waves in concrete, and obtained the modulus of elasticity by utilizing the wave propagation characteristics. Song et al. (2007, 2008) developed smart aggregates to perform structural health monitoring for concrete structures. The mortar-typed aggregate was embedded in concrete structures during casting and successfully used for monitoring damage in concrete structures by measuring an energy-based damage index. Dong et al. (2011) developed a cement-based piezoelectric ceramic composite and effectively applied it as a sensor for health monitoring of concrete structures. Hou et al. (2012, 2013) developed a marble-based smart aggregate for seismic compressive and shear stresses. Recently, Kee and Zhu (2013) developed PZT embedded sensors using a PZT disc that can be used as ultrasonic transmitting and receiving transducers for ultrasonic pulse velocity tests. In this study, the author developed a surface-mount sensor using a PZT disc for generating incident surface waves for surface wave velocity and transmission measurements in concrete structures.
The primary objective of this study is to investigate the feasibility of an innovative surface-mount sensor made of a PZT disc (hereafter refer to as “surface-mount sensor”) as a consistent source for surface wave velocity and transmission measurements in concrete structures. Two surface-mount sensors were attached to a concrete slab with dimensions of 1500 by 1500 by 200 mm (width X length X thickness). The slab possessed a surface-breaking crack located in the middle of the slab, which extended to varying depths. Two surface-mount sensors on either side of the surface-breaking crack worked as actuators driven by an ultrasonic pulse-and-receiver, and two accelerometers worked as a receiver. A series of surface wave measurements was performed to investigate the performance of the surface-mount sensors as a consistent source. An additional aspect to the investigation was to test the ability of surface-mount sensors to perform automated monitoring of an actual surface-breaking crack in a concrete beam subjected to various external loadings (compressive and flexural loading with stepwise increasing) (ElSafty and Abdel-Mohti 2013; Soltani et al. 2013).
2.1 Ultrasonic Surface Waves (USW) Method
The surface wave velocity methods involve determining the relationship between the wavelength and velocity of surface vibrations at varying vibration frequencies. The resulting relation is called a dispersion curve, which is generally determined by the spectral analysis of surface waves (SASW) (Nazarian and Desai 1993; Nazarian and Stokoe 1986). For plate-like concrete structures (deck, slab, wall, etc.), the ultrasonic surface wave (USW) technique has been demonstrated to be effective for evaluating material damages caused by many sources: ASR, DEF, freeze-and-thaw, and corrosion of reinforcing steel (Gucunski et al. 2013). The USW test consists of recording the response of the concrete, at two receiver locations, to an impact on the surface of the concrete in structures (See Fig. 1). The surface wave velocity can be obtained by measuring the phase difference ΔΦ between two different sensors (sensor 1 and sensor 2) (C = 2πfd/ΔΦ; where f is frequency, and d is the distance between two sensors). The frequency range of interest in the USW technique is a high-frequency range compared to the thickness of the tested object, in which surface waves are non-dispersive. In cases of relatively homogeneous materials, the velocity of the surface waves does not vary significantly with frequency. Therefore, the surface wave velocity can be precisely related to the elastic modulus of concrete, using the measured or assumed mass density and Poisson’s ratio of the material. A complex process called inversion is not necessary in the USW technique, leading to a substantially reduced time required for data interpretation and post-processing. In the case of a sound and homogenous concrete plate, the velocity of the surface waves will show little variability, while significant variation in the phase velocity will be an indication of the presence of a defect or other anomaly.
2.2 Surface Wave Transmission (SWT) Method
The surface wave transmission (SWT) method has been demonstrated to be effective for evaluating surface-breaking or sub-surface defects in concrete structures. The SWT method uses the frequency-dependent penetration depth of surface waves. When incident surface waves (R i ) propagate across a surface-breaking crack, the low-frequency components of the incident surface waves will transmit to the forward scattering field with attenuation (R tr ), while the high-frequency components will reflect back (R r ). Consequently, the transmission coefficient of surface waves Tr across a surface-breaking crack, which is defined as the ratio of spectral amplitudes of R tr to R i , depends on the frequency of surface waves and dimensions of the defect in the concrete. For example, an analytical solution relating Tr and the normalized crack depth (h/λ, h is the depth of a surface-breaking crack) was given by Achenbach and his colleagues (Achenbach et al. 1980; Angel and Achenbach 1984; Mendelsohn et al. 1980). It was demonstrated through numerical simulations and experimental studies that the SWT method is effective for evaluating the depth of surface-breaking cracks in concrete structures (Hevin et al. 1998; Kee 2011; Kee and Zhu 2011; Popovics et al. 2000; Shin et al. 2008; Song et al. 2003). Test configuration of the SWT method is the same as that of the USW method, consisting of two receivers and an impact source. However, the measured amplitude of the surface waves is sensitive to the coupling condition of the sensors and the magnitude of impact force, which may cause significant errors in predicted values. It has been demonstrated that the self-calibrating procedure is effective for eliminating undesirable effects due to sensors and sources in the SWT method (Popovics et al. 2000). Recently, Kee and Zhu (2011, 2010) proposed the air-coupled sensing method, which significantly improves signal consistency and test speed in transmission measurements of surface waves in concrete.
2.3 Preparation of Piezoelectric Sensor
A piezoelectric sensor is an active element that converts electrical energy to mechanical energy, and mechanical energy to electrical energy (i.e., the piezoelectricity). The piezoelectricity causes a sensor to produce electric charges when subjected to stress (receiving action) and conversely, to generate mechanical vibrations when an electrical voltage is applied (actuator action). In this study, a piezoelectric disc is used as an actuator for generating incident surface waves in concrete.
Two piezoelectric sensors were attached to the concrete surface to generate incident surface waves in concrete structures. The piezoelectric disc has a thickness of 0.2 mm (7.87 mils) and a diameter of 22 mm (866.14 mils). Two wires were then soldered to the electrodes on the piezoelectric disc. Since concrete surfaces are often under conductive environments, electrical shielding and waterproofing were needed. The waterproof procedure follows the description given by Jung (Jung 2005). First, five layers of polyurethane coating (M-coat A by VISHAY®) were applied to the surface of the piezoelectric discs. Each coating had to be fully air-dry before applying a subsequent one.
3 Experimental Program
3.1 Preparation of Specimens
In addition, a concrete beam (specimen 2) with dimensions of 400 by 1500 mm and a thickness of 190 mm was prepared in the laboratory, as shown in Fig. 5. Concrete used for the specimen 2 was normal weight, ready-mixed concrete made from Type I/II cement, river sand, and coarse aggregate with a maximum size of 19 mm. The design compressive strength of the concrete was 20 MPa. Three cylinder specimens were used to measure concrete compressive strength according to ASTM C39 (ASTM C39 2014), resulting in a measured compressive strength (at the time of testing) ranging from 22.3 to 25.58 MPa, with a mean value of 22.84 MPa. P-wave velocities measured with a pair of 54 kHz ultrasonic transducers were in the range of 4328 and 4375 m/s.
3.2 Surface Wave Measurements
In this study, five repeated signal data sets were collected at the same test location to investigate the repeatability of signals generated by the surface-mount sensors. The transmission coefficient measured from cracked regions was further normalized by the reference, producing the normalized transmission coefficient Tr n. It has been demonstrated that effects due to geometric attenuation and material damping can be effectively reduced by using the normalization process.
4 Results and Discussion
4.1 Repeatability of Signals Generated by the Surface-Mount Sensors
It is also observed that the presence of a surface-breaking crack decreases the magnitude of γ in high-frequency ranges: the useful frequency range becomes narrower as the depth of a crack increases from 0 to 50 mm (see Fig. 7b). However, it is not necessarily attributed to the coupling performance of the surface-mount sensor because the surface-breaking crack does not affect the surface-mount source and accelerometers. Instead, this phenomenon can be explained by the fact that the high-frequency components of surface waves are reflected back to the backward scattering field: consequently, the presence of a surface-breaking crack can significantly decrease the energy of transmitted surface waves in a higher frequency range.
4.2 Consistency of the Measured Surface Wave Parameters (Velocity and Transmission)
The consistency of measured results is of great interest when exploring the performance of surface-mount sensors. Ten repeated surface wave measurements were conducted at each test step using the same test setup and data acquisition system, in which surface-mount sensors located at A and D were alternatively used for generating incident waves in concrete. At each test step, the velocity and transmission coefficient of surface waves were determined by using Eqs. 1 and 2, respectively. In this study, the coefficient of variation (COV, the standard deviation divided by the mean value of a set of samples) was used as a means of consistency for the resulting surface wave velocity and transmission coefficients.
4.3 Sensitivity of the Measured Surface Wave Parameters to the Depth of a Surface-Breaking Crack
4.4 Effect of Crack Depth on Surface Wave Velocity Measurement
4.5 Application to Monitoring a Concrete Beam under Various Loadings
Loading history in test stages 1, 2, and 3 for specimen 2
Test stage 1
Test stage 1
Test stage 1
Test steps i
Test steps i
Test steps i
However, the effect of external loadings may pose difficulties in the interpretation of Tr n and C R . As shown in Fig. 12, Tr n and C R increase with an increase in the application of external compression P3. At the last loading step of P3, Tr n and C R were recovered to 90 and 95 %, respectively, of the values before cracking. Some portions of the incident surface waves (i.e., crack interfacial waves) are transmitted through the interface of an actual crack, which commonly has a partially closed interface. Increasing the compressive force gradually closes the concrete crack, increasing the interfacial stiffness of the crack. It was observed that both Tr n and C R are enhanced, owing to the crack interfacial waves, which may lead to substantial errors in predicting the depth of a surface-breaking crack in actual concrete structures.
One interesting finding is related to the potential for combining the results from the SWT and SASW as a more reliable crack depth estimation approach for testing actual structures. As observed in the theoretical results, the surface wave velocity is only sensitive to a crack deeper than about 80 % of the wavelength of surface waves. In addition, it was observed that the surface wave velocity is less sensitive to the interfacial stiffness of a surface-breaking crack than the surface wave transmission. Therefore, it is reasonable to say that some degradation of the surface wave velocity (about 10 %) across a surface-breaking crack is evidence of a deep crack, compared to the wavelength of surface waves.
The piezoelectric surface-mount sensors produce excellent signal coherence γ ≥ 0.999 in a wideband frequency range from 0 to 120 kHz. In contrast, signals generated by manual impacts have good signal consistency in a narrower frequency range of 10–30 kHz, and are dependent on the diameter of the steel ball. It was observed that the surface-mount sensors have consistent coupling under various stress states and damage levels of concrete.
Experimental variability in the transmission and velocity measurements measured using the piezoelectric surface-mount sensors are equivalent or less than that from manual impacts. The COVs of the TrPZT100 and CR,PZT100 are 1.24 ± 0.39 % (µ ± σ), and 0.04 ± 0.023 %, respectively, in a useful frequency range of 0–120 kHz. In contrast, the COVs of the TrSB and C R,SB are 2.29 ± 0.65 % (µ ± σ), and 0.3 ± 0.15 %, respectively, in a useful frequency range of 10 kHz to 30 kHz.
The proposed model for the surface wave transmission coefficient is demonstrated to be effective for evaluating the depth of a surface-breaking crack in concrete. However, special care is needed to apply the surface wave transmission method to a partially closed crack in actual concrete structure because of interference of the crack interfacial waves with transmitted surface waves.
Results from experiments and numerical simulations show that the surface wave velocity is only sensitive to cracks deeper than about 80 % of the wavelength of surface waves. However, it is reasonable to say that some degradation of the surface wave velocity (about 10 %) across a surface-breaking crack is evidence of a deep crack comparable to the wavelength of surface waves. Therefore, a fusion of the results from the SWT and SASW can be used as a more reliable crack depth estimation approach for testing actual structures.
This research was supported by a grant (15DRP-B066470-03) from Infrastructure and transportation technology promotion research Program funded by Ministry of Land, Infrastructure and Transport of Korean government.
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